Since nylon 12 has high chemical resistance, heat resistance and dimensional stability at the time of water absorption, it has been used as a material for injection moldings and for extrusion molds such as tubes, sheets and films in various industrial fields. In recent years, development of the use of nylon 12 has been making progress in the field of tubular molds such as fuel tubes, various hoses for industrial use, and gas pipes, and characteristics of nylon 12 required in this field have been highly advanced and diversified. Particularly, a demand has been increasing for a material for use in tubes, which has good moldability and shows excellent durability under severe environment, namely a member of nylon 12 which is possessed of excellent extrusion moldability, as well as excellent creep characteristics and fatigue characteristics.
With regard to the improvement of moldability of nylon 12, JP-A-7-278294 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), for example, discloses a method for the production of a member of nylon 12 which has excellent melt fluidity and is suited for precision molding. This type of nylon 12 is characterized in that its relative viscosity (to be referred to as ".eta..sub.r " hereinafter) and melt flow rate (to be referred to as "MFR" hereinafter) have a specific relationship. JP-A-7-278294 relates mainly to precision molding or the like injection molding. In general, injection molding shows a tendency in that the moldability becomes good as the melt fluidity of a polymer becomes superior, namely as the melt viscosity becomes low. In the case of the molding of tubes and the like extrusion moldings, however, too low melt viscosity causes a difficulty in obtaining tubular moldings having proper shapes because of the difficulty in keeping the tube shapes at the time of molding. Because of this, when the nylon 12 having superior melt fluidity disclosed in JP-A-7-278294 was used, there was limitation on molding conditions for obtaining proper tubular moldings. In order to produce good extrusion moldings stably, nylon 12 having a melt fluidity different from the case of injection molding was required. Because of this, development of nylon 12 suitable for extrusion molding is expected.
In addition, no prior art information is available concerning the improvement of creep characteristics and fatigue characteristics of nylon 12. It is said in general that the durability-related characteristics such as creep characteristics and fatigue characteristics of a polymer become excellent as its molecular weight is increased. There are some proposals concerning methods for increasing molecular weight of polyamide. For example, JP-A-3-97732 discloses a method in which a polyamide having relatively low molecular weight is mixed with a phosphorus compound under a melting condition and made into pellet, powder or the like shape and then its molecular weight is increased by solid phase polymerization. However, it is difficult to produce nylon 12 having a .eta..sub.r value of 5 or more even by the use of this method. Creep characteristics and fatigue characteristics of nylon 12 having a .eta..sub.r value of about 5 are not sufficient when compared with the intended values of the present invention. Also, being extremely low in melt fluidity, it was difficult to use nylon 12 having a .eta..sub.r value of 5 in extrusion molding. Because of this, concern has been directed toward the development of nylon 12 which has excellent extrusion moldability, creep characteristics and fatigue characteristics and is suitable as a tube material.
On the other hand, with the expansion of the range of their use, a demand has been increasing for nylon 12 which can be used under an environment where temperature changes widely or at a low temperature, e.g., -40.degree. C. However, flexibility, low temperature impact strength, durability-related creep characteristics and the like properties of the prior art nylon 12 were not satisfactory for its use under such severe environment.
With regard to the method for improving flexibility and low temperature impact strength of nylon 12, proposals have been made for example on a compsition which comprises nylon 12 and a plasticizer having good compatibility and a composition which comprises a polyamide resin, a plasticizer, a modified polyolefin and/or a thermoplastic elastomer.
With regard to the composition comprising a polyamide and a plasticizer, JP-A-62-283151, for example, discloses polyamide resin molds having flexibility which comprise nylon 12 and the like and 2-ethylhexyl-p-hydroxy benzoate as a plasticizer. Also, JP-A-1-185362 discloses a composition which comprises a polyamide resin and an ester as a plasticizer obtained from p-hydroxybenzoic acid and a branched alcohol having 12 to 22 carbon atoms.
However, according to these proposals, impact strength at a low temperature of for example -30.degree. C. or below was not sufficient and the creep characteristics was also insufficient, though flexibility was improved.
Also, with regard to the composition comprising a polyamide resin, a plasticizer and a modified polyolefin, JP-A-5-320504 discloses a composition which comprises a nylon 12 resin, a modified polyolefin composed of an olefin mainly consisting of ethylene and/or propylene and an .alpha.,.beta.-unsaturated carboxylic acid or a derivative thereof, and a plasticizer such as an ester synthesized for example from p-hydroxybenzoic acid and a branched alcohol having 12 to 22 carbon atoms. In addition, JP-A-8-325451 discloses tubular molds which comprise nylon 12, a modified polyolefin composed of an olefin mainly consisting of ethylene and/or propylene and an .alpha.,.beta.-unsaturated carboxylic acid or a derivative thereof and a plasticizer. These proposed compositions were not satisfactory in terms of their creep characteristics, though their flexibility and low temperature impact strength were improved.